The objective of this proposal is to study the cellular immunocompetence of the aging chimpanzee. Because of the close phylogenetic relationship between man and chimpanzee an understanding of this cellular immune system could be readily applied to the human immune system. We will determine in a group of aging chimpanzees their lymphocyte subpopulations, mitogen responsiveness and natural killer cell activity, and compare them to a group of younger animals. All these parameters have been shown to be low or absent in certain immunodeficiency states and appear to decline with increasing age. If data obtained from this pilot study demonstrate declining immunocompetence with increasing age, experimental immune intervention (i.e. immune enhancement) could be envisioned.